The regulation of enzyme levels in immature and neoplastic rat tissues is the broad purpose of these investigations. Enzymic differentiation of liver will be studied in explants of fetal rat liver cultured in vitro. Developmental changes in enzyme concentrations in vivo will be followed in brain with special reference to the changing enzymic composition of synaptosomes and of two populations (cytoplasmic and synaptosomal) of mitochondria. The growth rate and biochemical composition of tumors transplanted into immature versus adult hosts will be investigated together with the effects of these tumors on the biochemical differentiation of host organs.